Transgenic mouse expressing Prkag3

ABSTRACT

Transgenic non-human animals that express the γ3 subunit of PRKAG are described, as well as methods of using the transgenic non-human animals as models for improving treatment, prevention, or diagnosis of diseases related to energy metabolism, including obesity, dyslipidemia, insulin resistance syndrome, and type 2 diabetes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application under 35 U.S.C. § 371 and claims benefit under 35 U.S.C. § 119(a) of International Application No. PCT/IB03/00912 having an International Filing Date of Jan. 31, 2003, which claims the benefit of priority of U.S. Provisional Application Ser. No. 60/353,430 having a filing date of Feb. 1, 2002.

TECHNICAL FIELD

This invention relates to transgenic non-human animals expressing an AMP-activated protein kinase (AMPK) γ3 subunit, their use as models of studying human disease, and to methods of using these models for identifying compounds and compositions effective for the treatment of disease. In particular, the invention relates to transgenic non-human animals expressing the Prkag3 gene in skeletal muscle.

BACKGROUND

AMPK has a key role in regulating the energy metabolism in eukaryotic cells and is homologous to the SNF1 kinase in yeast (Hardie D. G., et al., 1998, Annu. Rev. Biochem. 67:821; Kemp B. E., et al., 1999, Trends. Biochem. Sci. 24(1):22–5). AMPK is composed of three subunits: the catalytic α-subunit and the two regulatory subunits β and γ. AMPK is activated by an increase in the ratio of AMP to ATP (AMP:ATP). Activated AMPK turns on ATP-producing pathways and inhibits ATP-consuming pathways. AMPK also can inactivate glycogen synthase, the key regulatory enzyme of glycogen synthesis, by phosphorylation (Hardie et al., 1998, supra). Several isoforms of the three different AMPK subunits are present in mammals. In humans, Prkaa1 and Prkaa2 encode the α1 and α2 subunits, Prkab1 and Prkab2 encode the β1 and β2 subunits, and Prkag1, Prkag2 and Prkag3 encode the γ1, γ2 and γ3 subunits, respectively.

Milan D., et al. (2001, Science, 288:1248–5) identified the nonconservative substitution of a glutamine for an Arginine (R225Q) in the Hampshire pig Prkag3 gene responsible for the dominant RN-phenotype (high glycogen content in skeletal muscle). Loss-of-function mutations in the homologous gene in yeast (SNF4) cause defects in glucose metabolism, including glycogen storage. Milan et al. further found that the expression of the Prkag3 gene is muscle-specific and that the AMPK activity in muscle extracts was about 3 times higher in normal rn+ pigs than in RN− pigs, both in the presence and absence of AMP. The distinct phenotype of the RN-mutation indicates that Prkag3 plays a key role in the regulation of energy metabolism in skeletal muscle.

AMPK is recognized as a major regulator of lipid biosynthetic pathways due to its role in the phosphorylation and inactivation of key enzymes such as acetyl-CoA carboxylase (ACC) (Hardie D. G., and Carling D., 1997, Eur. J. Biochem. 246:259–273). More recent data strongly suggest that AMPK has a wider role in metabolic regulation (Winder W. W., and Hardie D. G., 1999, Am. J. Physiol., 277:E1–E10): this includes fatty acid oxidation, muscle glucose uptake (Hayashi T., et al., 1998, Diabetes, 47:1369–1373; Merrill G. F., et al., Am. J. Physiol. 273:E1107–E1112; Goodyear L. J., 2000, Exerc. Sport Sci. Rev., 28:113–116), expression of cAMP-stimulated gluconeogenic genes such as PEPCK and G6Pase (Lochhead P. A., et al., 2000, Diabetes, 49:896–903), and glucose-stimulated genes associated with hepatic lipogenesis, including fatty acid synthase (FAS), Spot-14 (S14), and L-type pyruvate kinase (Foretz M., et al., 1998, J. Biol. Chem., 273:14767–14771). Chronic activation of AMPK may also induce the expression of muscle hexokinase and glucose transporters (Glut4), mimicking the effects of extensive exercise training (Holmes B. F., et al., 1999, J. Appl. Physiol. 87:1990–1995). Thus, it has been predicted that AMPK activation would be a good approach to treat type 2 diabetes (Winder et al., supra).

Zhou G, et al. (2001, J. Clin. Invest., 108:1167–1174) provided evidence that the elusive target of metformin's (a widely used drug for treatment of type 2 diabetes) actions is activated AMPK. In studies performed in isolated hepatocytes and rat skeletal muscles, metformin leads to AMPK activation, accompanied by an inhibition of lipogenesis (due to inactivation of acetyl-CoA carboxylase and suppression of lipogenic enzyme expression), suppression of the expression of SREBP-1 (a central lipogenic transcription factor), and a modest stimulation of skeletal muscle glucose uptake. Similar hepatic effects are seen in metformin-treated rats. Based on the use of a newly discovered AMPK inhibitor, their data suggest that the ability of metformin to suppress glucose production in hepatocytes requires AMPK activation.

SUMMARY

The invention is based on transgenic non-human animals expressing the AMPK γ3 subunit and their use as a model for diseases relating to energy metabolism, including obesity, dyslipidemia, insulin resistance syndrome, and type 2 diabetes. Such models can be used to improve diagnosis of diseases relating to energy metabolism as well as identifying and testing pharmaceutical compositions for better treatment and prevention of diseases relating to energy metabolism.

In one aspect, the invention features a transgenic non-human animal having integrated within its genome a nucleic acid encoding an AMP-activated protein kinase γ3 subunit or a variant thereof, wherein the nucleic acid is operably linked to a regulatory element. The nucleic acid can include a nucleotide sequence encoding a polypeptide having at least 75% sequence identity to the amino acid sequence set forth in SEQ ID NO:2. The nucleic acid can encode a polypeptide having an amino acid sequence selected from the group consisting of: (a) the amino acid sequence set forth in SEQ ID NO:2, (b) an R225Q variant of the amino acid sequence set forth in SEQ ID NO:2; (c) the amino acid sequence set forth in SEQ ID NO:4; and (d) an R225Q variant of the amino acid sequence set forth in SEQ ID NO:4. The nucleic acid can include a nucleotide sequence selected from the group consisting of (a) the nucleotide sequence set forth in SEQ ID NO:1; (b) a codon 225 variant of the nucleotide sequence set forth in SEQ ID NO:1; (c) the nucleotide sequence set forth in SEQ ID NO:3, (d) a codon 225 variant of the nucleotide sequence set forth in SEQ ID NO:3; (e) the nucleotide sequence set forth in SEQ ID NO:5; and (f) a nucleotide sequence corresponding to the mouse Prkag3 gene. The regulatory element can be a muscle specific regulatory element such as a myosin light chain promoter, a skeletal alpha actin promoter, a creatine kinase promoter, or an aldolase A promoter. The transgenic non-human animal can be selected from the group consisting of mice, rats, rabbits, cats, dogs, and pigs. Transgenic mice and pigs are particularly useful. Transgenic non-human animals can have an elevated glycogen content in skeletal muscle.

In another aspect, the invention features a transgenic non-human animal having a transgene integrated within its genome. The transgene includes a nucleotide sequence which hybridizes under stringent hybridization conditions (e.g., highly stringent) with a nucleic acid having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID NO:1 or a portion thereof, wherein the transgene acid is operably linked to a promoter that drives expression in skeletal muscle. The transgenic non-human animal can be selected from the group consisting of mice, rats, rabbits, cats, dogs, and pigs.

The invention also features an expression construct. The expression construct includes a regulatory element operably linked to a nucleotide sequence encoding a polypeptide having at least 75% sequence identity to the amino acid sequence shown in SEQ ID NO:2 or to a portion thereof; where the regulatory element is capable of mediating expression in skeletal muscle. The regulatory element can be muscle-specific regulatory element such as myosin light chain promoter, a myosin heavy chain promoter, a skeletal alpha actin promoter, a creatine kinase promoter, or an aldolase A promoter.

In yet another aspect, the invention features an expression construct that includes a regulatory element operably linked to a nucleotide sequence having at least 75% sequence identity to the nucleotide sequence shown in SEQ ID NO:1; where the regulatory element is capable of mediating expression in skeletal muscle. The regulatory element can be a muscle specific regulatory element as described above.

The invention also features a method for making a transgenic non-human animal having integrated within its genome a nucleic acid encoding an AMP activated protein kinase γ3 subunit or a variant thereof The nucleic acid is linked to a regulatory element that drives expression in skeletal muscle. The method includes introducing an expression construct described above into an ovum, an embryo, or embryonic stem cells of a non-human animal. The expression construct can be microinjected into the ovum or embryo of the non-human animal or into embryonic stem cells of the non-human animal. The expression construct can be electroporated into the embryonic stem cells.

In yet another aspect, the invention features a method of identifying a compound or composition effective for the treatment or prevention of a disease related to energy metabolism. The method includes (a) administering a test compound or test composition to a transgenic non-human animal described above; and (b) evaluating the effect of the test compound or test composition on the energy metabolism on the transgenic non-human animal; wherein the test compound or test composition is identified as effective for the treatment or prevention of the disease related to energy metabolism if energy metabolism is altered.

The invention also features a method of identifying a compound or composition effective for the treatment or prevention of diseases related to energy metabolism. The method includes (a) contacting a test compound or test composition with an organ, a tissue or cells derived from a transgenic non-human animal described above; and (b) evaluating the effect of the test compound or test composition on the energy metabolism on the organ, tissue or cells; wherein the test compound or test composition is identified as effective for the treatment or prevention of diseases related to energy metabolism if energy metabolism is altered. The tissue can be skeletal muscle. The cells can be muscle cells.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used to practice the invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of the linearized construct used for microinjection into mouse oocytes. Microinjection was done with two constructs separately, one construct included the wild-type mouse Prkag3 cDNA and the other construct included the mouse Prkag3 cDNA encoding the R225Q mutant.

FIG. 2 is a schematic depicting the primer locations (A, B, C) and exon-intron organization in the mouse Prkag3 amplicons used for genotyping the mice (from tail-tip genomic DNA) and for skeletal muscle mRNA expression analysis by RT-PCR.

FIG. 3 is a graph depicting the glycogen content vs the relative transgenic/endogenous Prkag3 mRNA expression ratio in skeletal muscle of 18 transgenic founder females.

DETAILED DESCRIPTION

The invention relates to transgenic non-human animals that express the AMPK γ3 subunit and methods of using the animals for the development of drugs for the treatment or prevention of diseases related to energy metabolism, such as obesity, dyslipidemia, insulin resistance syndrome, and type 2 diabetes. Without being bound by a particular mechanism, modulation of the amount or activity of the γ3 subunit of AMPK, a major cellular regulator of lipid and glucose metabolism, may be beneficial in the treatment of such energy metabolism diseases. An increase in overall activity of AMPK in muscle can increase levels of glycogen, which is coupled to increased glucose uptake and lowered blood glucose levels.

As used herein, “transgenic non-human animal” includes the founder transgenic non-human animals and progeny of the founders, as well as cells and tissues from such animals. Transgenic non-human animals can be farm animals such as pigs, goats, sheep, cows, horses, and rabbits, rodents such as rats, guinea pigs, and mice, and non-human primates such as baboons, monkeys, and chimpanzees. Transgenic pigs and mice are particularly useful.

A transgenic non-human animal of the invention contains a nucleic acid encoding an exogenous AMPK γ3 subunit (e.g., a human, mouse, or pig AMPK γ3 subunit) integrated within its genome. As used herein, the term “AMPK γ3 subunit” refers to a polypeptide having at least 200 amino acids (e.g., at least 300 or 400 amino acids) of the full-length polypeptide. In some embodiments, the AMPK γ3 subunit is full-length. The AMPK γ3 subunit can be wild-type or can be a variant (e.g., the R225Q variant). The cDNA encoding the human γ3 subunit has been cloned and characterized (WO 01/20003; Milan et al., 2001, supra; GenBank Accession Nos. AF214519 and AF249977; Cheung P. C., et al., 2000, Biochem J., 346: 659–69). Genetic variants of the human Prkag3 gene encoding the AMPK γ3 subunit have been identified (WO 01/77305). The mouse sequence encoding the AMPK γ3 subunit is provided in SEQ ID NO:3.

In some embodiments, the exogenous nucleic acid can encode a polypeptide having at least 75% (e.g., at least 80%, 85%, 90%, 95%, 98%, or 99%) sequence identity to the amino acid sequence shown in SEQ ID NO:2 or SEQ ID NO:4, or to a fragment of SEQ ID NO:2 or SEQ ID NO:4 at least 200 amino acids in length. The nucleic acid molecule can encode the amino acid sequence of SEQ ID NO:2 or SEQ ID NO:4, an R225Q variant of SEQ ID NO:2 or SEQ ID NO:4, or fragments of such polypeptides that are at least 200 amino acids in length.

In other embodiments, the exogenous nucleic acid includes a nucleotide sequence having at least 75% sequence identity (e.g., at least 80%, 85%, 90%, 95%, 98%, or 99%) to the nucleotide sequence shown in SEQ ID NO:1 or SEQ ID NO:3, or to a fragment of SEQ ID NO:1 or SEQ ID NO:3 at least 600 nucleotides in length (e.g., at least 900 or 1200 nucleotides in length). In some embodiments, the nucleic acid includes the nucleotide sequence set forth in SEQ ID NO:1 or SEQ ID NO:3, a codon 225 variant (e.g., R225Q variant) of the nucleotide sequences set forth in SEQ ID NO:1 or SEQ ID NO:3, or a fragment of such nucleic acids at least 600 nucleotides in length.

Percent sequence identity is calculated by determining the number of matched positions in aligned nucleic acid sequences, dividing the number of matched positions by the total number of aligned nucleotides, and multiplying by 100. A matched position refers to a position in which identical nucleotides occur at the same position in aligned nucleic acid sequences. Percent sequence identity also can be determined for any amino acid sequence. To determine percent sequence identity, a target nucleic acid or amino acid sequence is compared to the identified nucleic acid or amino acid sequence using the BLAST 2 Sequences (Bl2seq) program from the stand-alone version of BLASTZ containing BLASTN version 2.0.14 and BLASTP version 2.0.14. This stand-alone version of BLASTZ can be obtained from Fish & Richardson's web site (world wide web at fr.com/blast) or the U.S. government's National Center for Biotechnology Information web site (world wide web at ncbi.nln.nih.gov). Instructions explaining how to use the Bl2seq program can be found in the readme file accompanying BLASTZ.

Bl2seq performs a comparison between two sequences using either the BLASTN or BLASTP algorithm. BLASTN is used to compare nucleic acid sequences, while BLASTP is used to compare amino acid sequences. To compare two nucleic acid sequences, the options are set as follows: -i is set to a file containing the first nucleic acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second nucleic acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastn; -o is set to any desired file name (e.g., C:\output.txt); -q is set to −1; -r is set to 2; and all other options are left at their default setting. The following command will generate an output file containing a comparison between two sequences: C:\Bl2seq -i c:\seq1.txt -j c:\seq2.txt -p blastn -o c:\output.txt -q −1 -r 2. If the target sequence shares homology with any portion of the identified sequence, then the designated output file will present those regions of homology as aligned sequences. If the target sequence does not share homology with any portion of the identified sequence, then the designated output file will not present aligned sequences.

Once aligned, a length is determined by counting the number of consecutive nucleotides from the target sequence presented in alignment with sequence from the identified sequence starting with any matched position and ending with any other matched position. A matched position is any position where an identical nucleotide is presented in both the target and identified sequence. Gaps presented in the target sequence are not counted since gaps are not nucleotides. Likewise, gaps presented in the identified sequence are not counted since target sequence nucleotides are counted, not nucleotides from the identified sequence.

The percent identity over a particular length is determined by counting the number of matched positions over that length and dividing that number by the length followed by multiplying the resulting value by 100. For example, if (1) a 1000 nucleotide target sequence is compared to the sequence set forth in SEQ ID NO:1, (2) the Bl2seq program presents 850 nucleotides from the target sequence aligned with a region of the sequence set forth in SEQ ID NO:1 where the first and last nucleotides of that 850 nucleotide region are matches, and (3) the number of matches over those 850 aligned nucleotides is 750, then the 1000 nucleotide target sequence contains a length of 850 and a percent identity over that length of 88 (i.e., 750) 850×100=88).

It will be appreciated that different regions within a single nucleic acid target sequence that aligns with an identified sequence can each have their own percent identity. It is noted that the percent identity value is rounded to the nearest tenth. For example, 78.11, 78.12, 78.13, and 78.14 are rounded down to 78.1, while 78.15, 78.16, 78.17, 78.18, and 78.19 are rounded up to 78.2. It also is noted that the length value will always be an integer.

It is contemplated that it may be useful to include intron sequences in a nucleic acid encoding an AMPK γ3 subunit or a variant thereof. For example, one or more of the intron sequences present in the Prkag3 gene shown in SEQ ID NO:5 can be included in the nucleic acid. It is likely that not all of the intron sequences are necessary and that intron sequences from Prkag3 from other species or intron sequences from genes coding for other protein may also be suitable and can be inserted into the nucleotide sequence coding for the γ3 subunits of AMPK in a suitable manner.

Nucleic acid useful in the invention will generally hybridize under stringent conditions with the sequence complementary to the nucleotide sequence of SEQ ID NO:1 or a fragment thereof at least 600 nucleotides in length. Thus, a transgenic non-human animal of the invention can have integrated within its genome, a nucleic acid that hybridizes with the complementary sequence to the nucleotide sequence of SEQ ID NO:1 or a part thereof under stringent hybridization conditions. Suitable nucleic acids can hybridise under highly stringent hybridization conditions. The term “stringent” when used in conjunction with hybridization conditions is as defined in the art, i.e., 15–20° C. under the melting point Tm. Preferably the conditions are highly stringent”, i.e., 5–10° C. under the melting point Tm. High stringency conditions can include the use of low ionic strength buffer and a high temperature for washing, for example, 0.015 M NaCl/0.0015 M sodium citrate (0.1×SSC), 0.1% sodium dodecyl sulfate (SDS) at 65° C. Alternatively, denaturing agents such as formamide can be employed during hybridization, e.g., 50% formamide with 0.1% bovine serum albumin/0.1% Ficoll/0.1% polyvinylpyrrolidone/50 mM sodium phosphate buffer at pH 6.5 with 750 mM NaCl, 75 mM sodium citrate at 42° C. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Molecular Cloning: A Laboratory Manual, 3rd ed., Sambrook et al. eds., Cold Spring Harbor Laboratory Press, 2001; DNA Cloning: A practical Approach, Glover & Hames eds., Oxford University Press, 1996; Nucleic Acid Hybridization: Essential techniques, Ross ed. Wiley, 1998.

In transgenic non-human animals of the invention, the nucleic acid is operably linked to a regulatory element that can promote expression in muscle. As used herein, the term “operably linked” refers to the placement of the regulatory element and nucleic acid in such a manner that the nucleic acid is transcribed. The regulatory element can be a skeletal muscle specific promoter, such as a myosin light chain promoter, a myosin heavy chain promoter, a skeletal alpha actin promoter, a creatine kinase promoter, or an aldolase A promoter.

The invention also features expression constructs suitable for generating transgenic non-human animals of the invention. The expression constructs can include a promoter capable of mediating expression in skeletal muscle operably linked to a nucleic acid encoding an AMPK γ3 subunit as described above.

Various techniques known in the art can be used to introduce expression constructs into non-human animals to produce the founder lines of the transgenic non-human animals. Such techniques include, but are not limited to, pronuclear microinjection (U.S. Pat. No. 4,873,191), retrovirus mediated gene transfer into germ lines (Van der Putten et al., Proc. Natl. Acad. Sci. USA, 82:6148, 1985), gene targeting into embryonic stem cells (Thompson et al., Cell, 56:313, 1989), electroporation of embryos (Lo, Mol. Cell. Biol., 3:1803, 1983), and transformation of somatic cells in vitro followed by nuclear transplantation (Wilmut et al., Nature, 385(6619):810–813, 1997; and Wakayama et al., Nature, 394:369–374, 1998).

In a preferred embodiment, the expression construct is microinjected into an ovum or embryo of the non-human animal or into embryonic stem cells of the non-human animal.

Once transgenic non-human animals have been generated, expression of the AMPK γ3 subunit can be assessed using standard techniques. Initial screening can be accomplished by Southern blot analysis or PCR techniques to determine whether or not integration of the transgene has taken place. See, for example, sections 9.37–9.52 of Sambrook et al., 1989, “Molecular Cloning, A Laboratory Manual,” second edition, Cold Spring Harbor Press, Plainview; N.Y., for a description of Southern analysis.

Expression of the nucleic acid encoding AMPK γ3 subunit in the tissues of the transgenic non-human animals can be assessed using techniques that include, but are not limited to, Northern blot analysis of tissue samples obtained from the animal, in situ hybridization analysis, and reverse-transcriptase PCR (RT-PCR).

Methods of Using Transgenic Non-Human Animals

As discussed above, transgenic non-human animals according to the invention can be used as a model for diseases related to energy metabolism, such as obesity, dyslipidemia, insulin resistance syndrome and type 2 diabetes. In particular, transgenic non-human animals of the invention can be used to identify a compound or composition effective for the treatment or prevention of diseases related to energy metabolism. Compounds or compositions can be identified by administering a test compound or composition to a transgenic non-human animal of the invention or by contacting the test compound or composition with an organ, a tissue (e.g., skeletal muscle) or cells (e.g., muscle cells) derived from the transgenic non-human animal. Effects of the test compound or composition on the energy metabolism on the transgenic non-human animal, organ, tissues or cells are evaluated. For example, glycogen content can be assessed in the transgenic non-human animals. Test compounds or compositions that alter energy metabolism can be effective for the treatment or prevention of diseases related to energy metabolism.

Test compounds can be formulated into pharmaceutical compositions by admixture with pharmaceutically acceptable non-toxic excipients or carriers and administered to transgenic non-human animals of the invention by any route of administration. For example, parenteral routes such as subcutaneous, intramuscular, intravascular, intradermal, intranasal, inhalation, intrathecal, or intraperitoneal administration, and enteral routes such as sublingual, oral, or rectal administration can be used.

The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.

EXAMPLES Example 1

Isolation and characterization of the human Prkag3 gene: The published cDNA sequences encoding the human AMPK γ3 subunit (Genbank accession nos. AJ249977 and AF214519) (SEQ ID NO:1) were used to search the database for genomic sequences comprising the human Prkag3 gene and promoter. The human BAC clone RP11-459I19 (Genbank accession No. AC009974) was identified and found to comprise the complete Prkag3 gene (SEQ ID NO:5). The coding part of the gene contains at least 14 exons and spans more than 8 kb. The 5′ end of the reported cDNA sequence (AJ249977) consists a donor-acceptor splice signal indicating the possible presence of yet another exon in the 5′ end of the gene.

Example 2

Cloning and in vitro mutagenesis of the mouse Prkag3 coding sequence: Genomic mouse Prkag3 sequence was obtained by sequencing a PRKAG3-positive clone isolated from a BAC library of mouse genomic DNA. The coding sequence was deduced from this genomic sequence with presumed start and stop codons in concordance with the human cDNA sequence in GenBank AJ249977. The mouse sequence was used to design primers for RT-PCR amplification of the complete coding Prkag3 sequence from a mouse skeletal muscle poly A RNA sample (Clontech, Palo Alto, Calif.). The mouse forward 5′ CACC ATG GAG CCC GAG CTG GAG CA (SEQ ID NO:7) and reverse 5′ GTC TCA GGC GCT GAG GGC ATC (SEQ ID NO:8) primer sequences include the translation start and stop codons, respectively (in bold). The forward primer also includes four additional bases (in italics) at the 5′ end to facilitate translation initiation. Reverse transcription was performed on 200 ng mouse skeletal muscle mRNA using the First-Strand cDNA Synthesis Kit (Amersham Pharmacia Biotech, Little Chalfont Buckinghamshire, UK) with random hexarner priming in a 15 μl reaction volume. The resulting product was used for PCR at a 1:6 dilution at standard conditions with primer annealing at 63° C. The RT-PCR product (˜1.5 kbp) was gel purified and ligated into the pCRII TA TOPO cloning vector (Invitrogen, Groningen, Netherlands). Ten clones were sequenced and the consensus sequence set forth in SEQ ID NO:3 was identical to the coding Prkag3 sequence derived from the mouse BAC. A clone of this consensus sequence was selected for inclusion of the insert in the transgenic construct and for introduction of a mutation, R225Q, corresponding to the porcine RN⁻ mutation (Milan et al., supra). The R225Q mutation was introduced by in vitro mutagenesis of the nucleotides AG to CA at positions 685–686 in the mouse sequence given in SEQ ID NO:3, which changes the codon AGG for arginine (R) to CAG for glutamine (Q). This mutagenesis was carried out using the Quikchange Site-Directed Mutagenesis Kit (Stratagene, La Jolla, Calif.) with forward and reverse primers over the GTG GCC AAC GGT GTG CAG GCA GCT CCT CTG TGG (SEQ ID NO:9) sequence (mutagenesis site in bold).

Example 3

Transgene constructs and microinjection: The wild-type and R225Q inserts in the pCRII TA TOPO cloning vector from Example 2 were removed using an Eco RI digest. These inserts then were ligated into the pMLC vector (Gros et al. 1999) kindly provided by Dr Fatima Bosch. The resulting transgene contained the complete mouse Prkag3 coding sequence (with attached Kozak element as described above) flanked by the myosin light chain 1 (MLC1) promoter on the 5′ end and the SV40 untranslated region (with a small intron and a polyA site) as well as the MLC1/3 enhancer on the 3′ end (FIG. 1). The MLC1 promoter is expected to direct the expression primarily to white (fast twitch) skeletal muscle fibers. This transgene was removed from the plasmid using a NotI/XhoI double digestion and gel-purified on an agarose gel, without exposure to UV light or ethidium bromide. Wild type and R225Q mutated forms of the transgene were used for microinjection into mouse oocytes (CBA×C57B1/6J) at the Mouse Camp facility at Karolinska Institute in Stockholm (world wide web at mousecamp.ki.se).

Example 4

Genotyping and mRNA expression analysis: The founders were tested for transgene incorporation using a PCR test with a forward primer in Prkag3 exon 12 (5′ GCT GCC CAG CAA ACC TAC AAC) (SEQ ID NO:10) and two alternative reverse primers located in the mouse Prkag3 3′UTR (5′ AAG ATG GCT TGG GTG TGA GGA C) (SEQ ID NO:11); not included in the construct, and SV40 3′UTR (5′ TGC TCC CAT TCA TCA GTT CCA TAG) (SEQ ID NO:12), respectively (FIG. 2). The expected PCR product sizes are shown in Table 1.

TABLE 1 Expected PCR results using the forward primer in mouse Prkag3 exon 12 and reverse primers in the 3′UTR of mouse Prkag3 and SV40, respectively. PCR product size (bp) Gene Genomic DNA cDNA Endogenic 617 287 PRKAG3 Transgenic 453 387 PRKAG3

Genomic DNA was prepared from mouse tails according to a standard protocol and used in 10 μl reactions including 0.35 U AmpliTaq DNA polymerase (Perkin Elmer, Branchburg, N.J., USA), 1×PCR buffer, 1.5 mM MgCl₂, 0.2 MM of each dNTP, 2.5 pmol of each primer and 5% DMSO. Thermocycling was carried out using a PTC 200 instrument (MJ Research, Watertown, Mass., USA) and included 40 cycles with annealing at 58° C. for 30 s and extension at 72° C. for 1 min. The denaturation steps were at 95° C. for 1–2 min in the first two cycles, and at 94° C. for 1 min in the remaining cycles. The same set of primers was used for RT-PCR amplification of the corresponding cDNA fragments from quadriceps mRNA samples (FIG. 2, Table 1). Quadriceps mRNA was prepared using the Quickprep Micro mRNA Purification Kit (Amersham Pharmacia Biotech, Little Chalfont Buckinghamshire, UK) and used for first cDNA strand synthesis as described above. The resulting product was used for PCR at a 1:6 dilution with conditions in essence the same as for the PCR on tail genomic DNA described above. This simultaneous amplification of transgenic and endogenous Prkag3 cDNA was used for estimation of the relative transgenic/endogenous Prkag3 mRNA expression.

Example 5

Glycogen measurements: The glycogen content was measured on quadriceps samples from 18 transgenic founder females at 10–11 weeks of age. Six of these founders were from the microinjection of the Prkag3 wild type construct whereas the other 12 founders were from the microinjection of the R225Q construct.

Among the 18 female founders tested, both constructs showed a clear association between transgenic mRNA expression and elevated glycogen levels in skeletal muscle. The most dramatic effects on glycogen levels were observed among mice with the mutated construct (FIG. 3).

OTHER EMBODIMENTS

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. 

1. A transgenic mouse having integrated within its genome a nucleic acid encoding an AMP-activated protein kinase γ3 subunit having at least 85% sequence identity to the sequence set forth in SEQ ID NO:2, wherein said nucleic acid is operably linked to a regulatory element, and wherein said transgenic mouse has an elevated glycogen content in skeletal muscle.
 2. The transgenic mouse according to claim 1, wherein said nucleic acid comprises a nucleotide sequence encoding a polypeptide having at least 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:2.
 3. The transgenic mouse according to claim 2, wherein said nucleic acid encodes a polypeptide having an amino acid sequence selected from the group consisting of: (a) the amino acid sequence set forth in SEQ ID NO:2, and (b) an R225Q variant of the amino acid sequence set forth in SEQ ID NO:2.
 4. The transgenic mouse according to claim 1, said nucleic acid comprising a nucleotide sequence selected from the group consisting of (a) the nucleotide sequence set forth in SEQ ID NO:1; (b) a codon 225 variant of the nucleotide sequence set forth in SEQ ID NO:1; and (c) the nucleotide sequence set forth in SEQ ID NO:5.
 5. The transgenic mouse according to claim 1, wherein said regulatory element is a muscle specific regulatory element.
 6. The transgenic mouse according to claim 5, wherein said muscle specific regulatory element is selected from the group consisting of a myosin light chain promoter, a skeletal alpha actin promoter, a creatine kinase promoter, and an aldolase A promoter.
 7. A transgenic mouse having a transgene integrated within its genome, said transgene having a nucleotide sequence which hybridizes under highly stringent hybridization conditions with a nucleic acid having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID NO: 1 or a portion thereof, said sequence encoding an AMP activated protein kinase γ3 subunit or an R225 variant thereof, wherein said transgene is operably linked to a promoter that drives expression in skeletal muscle, and wherein said transgenic mouse has an elevated glycogen content in skeletal muscle.
 8. A method for making a transgenic mouse having integrated within its genome, a nucleic acid encoding an AMP activated protein kinase γ3 subunit or an R225 variant thereof, wherein said nucleic acid is linked to a regulatory element that drives expression in skeletal muscle, said method comprising introducing an expression construct into an ovum or an embryo of a mouse, said expression construct comprising said regulatory element operably linked to a nucleotide sequence encoding a polypeptide having at least 85% sequence identity to the amino acid sequence shown in SEQ ID NO:2 or to a portion thereof, and wherein said transgenic mouse has an elevated glycogen content in skeletal muscle.
 9. The method according to claim 8, where the expression construct is microinjected into said ovum or embryo of the mouse.
 10. A method of identifying a compound or composition effective for altering energy metabolism, said method comprising: (a) administering a test compound or test composition to said transgenic mouse of claim 1; and (b) evaluating the effect of said test compound or test composition on the energy metabolism on said transgenic mouse; wherein said test compound or test composition is identified if energy metabolism is altered.
 11. A method for making a transgenic mouse having integrated within its genome, a nucleic acid encoding an AMP activated protein kinase γ3 subunit or an R225 variant thereof, wherein said nucleic acid is linked to a regulatory element that drives expression in skeletal muscle, said-method comprising introducing an expression construct into an ovum or an embryo, of a mouse, said expression construct comprising said regulatory element operably linked to a nucleotide sequence having at least 85% sequence identity to the nucleotide sequence shown in SEQ ID NO: 1, and wherein said transgenic mouse has an elevated glycogen content in skeletal muscle.
 12. The method according to claim 11, where the expression construct is microinjected into said ovum or embryo of the mouse.
 13. A method for making a transgenic mouse having integrated within its genome, a nucleic acid encoding an AMP activated protein kinase γ3 subunit or an R225 variant thereof, said method comprising introducing an expression construct into embryonic stem cells of a mouse, said expression construct comprising a regulatory element operably linked to a nucleotide sequence having at least 85% sequence identity to a nucleotide sequence encoding a polypeptide having at least 85% sequence identity to the amino acid sequence shown in SEQ ID NO: 2 or to a portion thereof; where said regulatory element is capable of mediating expression in skeletal muscle, and wherein said transgenic mouse has an elevated glycogen content in skeletal muscle.
 14. The method according to claim 13, where the expression construct is electroporated into said embryonic stem cells. 